As a formed lid which can be fitted to a cup-shaped container having a flange section, a drop lid-shaped one is known which generally comprises a side wall suspending downwardly from the inner edge of a flange section, and a top panel formed at the lower end of the side wall.
For such a combination of the formed lid and the cup-shaped container, it is common practice to subject the container, which is transported by a conveyor or the like, to a process including a step of filling contents into the container, a step of supplying the formed lid to the container, a step of fitting the formed lid and the container together, and a sealing step, thereby sealing the container, while stopping the container in each step in order to perform the operations intermittently.
For the sealing of the formed lid and the cup-shaped container in the above-mentioned sealing step, welding by heat sealing is generally widely used, because it is a convenient measure. Welding by heat sealing, however, takes time for heat fusion and subsequent cooling, and is not fully satisfactory in terms of productivity. A general heat sealing method using a heat sealing bar, moreover, requires that heat be conducted from the outer surface of a welding region to a surface to be sealed. In a thick-walled container or the like, heat conduction takes time, and productivity decreases. Thus, there are limitations on the wall thickness, and the problem arises that the degrees of freedom of the container and lid shapes are low. Furthermore, a predetermined time is taken until the heat sealing area is cooled and completely closed. In the case of filling contents having a self-generated pressure or hot filling, in particular, a gas in a head space thermally expanded by sealing heat escapes from the sealing region in a molten state, thereby posing the possibility of seal peeling.
As a method of welding packaging members, such as a container and a lid, on the other hand, welding by laser has also been known so far. Patent Document 1, for example, proposes that a bottom cover and an upper lid be welded to a container body by laser welding for integration. In such welding of packaging members by laser welding, the seal interface is welded without great restrictions on the wall thicknesses of the members, and the time required for welding is shortened in comparison with heat sealing. Thus, the productivity is improved compared with the heat sealing method. Besides, laser welding enables continuous welding operations. Thus, laser welding can be performed without the need to stop the container once in the sealing step (laser welding step), so that a further increase in productivity can be achieved.